The recordable CD (so-called “CD-R”) is known as an information recording medium (optical disk) on which information can be recorded just once with a laser beam. It affords an advantage in that smaller quantities of CDs can be provided at more reasonable cost and more rapidly than when manufacturing conventional CDs. Demand for it has increased as personal computers have become widespread. The typical configuration of a CD-R information recording medium comprises a transparent disklike substrate on which are sequentially stacked a recording layer comprised of an organic dye, a reflective layer comprised of a metal such as gold, and a protective layer made of resin. Data are recorded on an optical disk by irradiating a near infrared laser beam (normally, a laser beam with a wavelength in the vicinity of 780 nm) onto the recording layer, causing it to undergo localized thermal deformation. Data are read (reproduced) by irradiating a laser beam of the same wavelength as the laser beam for recording and detecting differences in the reflectance of portions of the recording layer that have been thermally deformed (recorded portions) and portions that have not been deformed (unrecorded portions).
As the performance of personal computers has improved and the speed of the Internet has increased in recent years, image data (particularly animation and the like) comes to be handled, requiring higher capacity information recording media. An optical disk known as a recordable digital variable disk (so-called “DVD-R”), in which a narrowed laser beam of shorter wavelength is used to increase the recording density, is being sold as information recording media of higher recording density. This optical disk is manufactured in a configuration comprising two disks, each of which sequentially comprises a transparent disklike substrate that is 120 mm or 80 mm in diameter, on which are formed pregrooves with a track pitch of 0.8 micrometer, which is narrower than the 1.6 micrometers of a CD-R; a recording layer comprising dye; and, normally, a reflective layer and a protective layer over the recording layer. Alternatively, the configuration may comprise such a disk, adhered by means of adhesive to a disklike protective substrate of roughly identical size, with the recording layer to the inside. Recording and reproduction on a DVD-R are conducted by irradiation with a visible laser beam (normally, a laser beam with a wavelength falling within a range of 600 nm to 700 nm), permitting higher density recording than is possible with a CD-R optical disk. An optical disk with specifications similar to those of the DVD-R is now being sold as the DVD+R.
The recordable DVD information recording medium permits the recording of several times the quantity of data of a conventional CD-R. Since high recording sensitivity and, in particular, the rapid processing of large amounts of information are required, a low error rate is desirable in high-speed recording. The recording layer comprising dye generally has low stability over time to heat or light. Thus, the development of a recording layer capable of maintaining stable performance for extended periods to heat or light is desirable.
In a recordable DVD, the reduction in recording time, that is, the increase in recording speed, is desirable as in a CD-R. There is a need for the recordable DVD to increase the sensitivity and improve the decreasing writing precision (deterioration in the form of jitter) due to increase the writing laser power.
Japanese Unexamined Patent Publication (KOKAI) Showa No. 63-209995 [Patent Reference 1] discloses a CD-R information recording medium in which a recording layer comprised of an oxonol dye is provided on a substrate. The use of such a dye compound is described as maintaining stable recording reproduction characteristics for an extended period, and an oxonol dye compound into which ammonium is introduced in the form of a salt is described. Japanese Unexamined Patent Publication (KOKAI) Heisei No. 10-109475 [Patent Reference 2] describes bipyridinium as being effective in enhancing resistance to light as an ammonium introduced in the form of a salt. It is further stated that the dye is not limited to oxonol, and that a variety of dyes are effective. Japanese Unexamined Patent Publication (KOKAI) Heisei No. 10-297103 [Patent Reference 3] describes that an oxonol dye having a biologen-paired salt is effective. Japanese Unexamined Patent Publication (KOKAI) No. 2000-52658 [Patent Reference 4] and Japanese Unexamined Patent Publication No. 2002-249674 [Patent Reference 5] describe oxonol dye compounds exhibiting high resistance to light and durability and providing optical information recording media with good recording characteristics. Japanese Unexamined Patent Publication (KOKAI) No. 2002-59652 [Patent Reference 6] describes a technique of mixing two dyes of differing sensitivity, with oxonol dyes among them. Japanese Unexamined Patent Publication (KOKAI) No. 2004-188968 [Patent Reference 7] discloses bis-oxonol dyes of specific structure.